Bispectral analysis applied for measurement of nonlinear characteristics of vibration propagation in soft tissues

A new method of measurement of nonlinear propagation characteristics of low frequency vibration in the tissues by using bispectral analysis is proposed. The main nonlinear characteristic of low frequency vibration in soft tissue can be represented as amplitude ratio of 3rd harmonics to the fundamental frequency components defined by the parameter N. The evolution of N as function of amplitude of low frequency vibration at an observing point in the interested region of tissue is observed, and the parameter of nonlinearity of medium is estimated based on the formulation. For measurement of N the bispectral analysis was adopted so that signal to noise ratio is increased in connection with the ultrasonic pulse Doppler method. Experimental results for agar phantom and pig tissue (pork) are shown.